The descending systems comprised of medullary reticulospinal and lateral vestibulo-spinal neurons have been shown to be responsible for facilitating the execution of a mammalian behavior which has been proven to be unusually accessible to cellular and molecular analysis: the estrogen-progestin dependent reproductive behavior, lordosis. Contraction of the deep back muscles to an extent seen in no other behavior, upon somatosensory stimulation on specific regions of skin, is the neuromuscular basis of lordosis. The studies proposed here logically extend previous electrophysiological and behavioral work in our lab. I. Having already established reticulospinal and lateral vestibulospinal control over the deep back muscles lateral longissimus, medial longissimus and transverso-spinalis, we will determine the sensory ascending and brainstem descending neurons which project to the reticulospinal and vestibulospinal neuronal groups responsible. Using microstimulation with currents of 30mualpha or less, we will determine effective sites for the activation of deep back muscles, and use a low- diffusion method of application of retro-grade tracers developed in our lab to see which neurons are projecting to these sites. II. Since damage to the ventromedial nucleus of the hypothalamus can reduce lordosis behavior, we will use large ventromedial hypothalamic lesions (electrolytic or by excitotoxin microinjection) and electrical stimulation, as well as control lesions and stimulation, to discover some of the ways in which medial hypothalamic neurons can influence the ability of reticulospinal and vestibulospinal neurons to facilitate, in turn, deep back muscle motoneurons. Here, and throughout this project, we will use electromyographic (EMG) recording to advantage: it is a productive approach to measuring the output of the motoneuronal pools for deep back muscles. III. Since several hypothalamic neuropeptides have already been characterized with respect to their effects on lordosis behavior, it will be interesting to study their ability to influence the activation of deep back muscle motoneurons by reticulospinal or vestibulospinal neurons. For example, the decapeptide LHRH, known to facilitate reproductive behavior, can be contrasted to beta-endorphin, which decrease lordosis. Work with chronically prepared rats, throughout, will allow comparisons between electromyographic and behavioral observations, while larger numbers of parametric studies can be done efficiently in urethane- anesthetized animals. in the freely moving animals, comparisons of EMG activation across a variety of behavioral situations will reveal those patterns of activity which are specific to the extremely dorsiflexed posture of lordosis. Traditionally in neurobiology, it has been difficult to follow the mechanisms by which forebrain neurons could transmit signals through the brainstem reticular formation, to influence behaviors which are executed through motoneurons located in the spinal cord. These experiments extend our approach to discovering how hypothalamic neurons might influence one, concrete hormone-dependent reproductive behavior.